EPROMs such as the 27C1024 series are 16 bits wide. The data in S4's memory is 8
bits wide, so each EPROM word takes two bytes, from an even and an odd address.
There are two ways to do this - "Low-High", in which even bytes are written to D7-D0
and odd bytes to D15-D8 of the EPROM word, and "High-Low", in which even bytes are
written to D15-D8 and odd bytes to D7-D0. Intel users customarily prefer the former
arrangement, Motorola users the latter.
S4 defaults to the "Low-High" arrangement. If you select a 16-bit EPROM and press
Esc, this is what you get. If you press Enter, you can switch between Low-High and
High-Low using the left and right arrow keys.
You can, if you prefer, make S4 default to "High-Low" mode by editing a single byte
in the library ROM. For 40-pin library version 1.04, setting address 958Bh to 00
causes S4 to default to High-Low mode. For 16-bit library version 1.10, setting
address 9BDAh to 00 causes S4 to default to High-Low mode. Setting it to 01, the
factory default, causes S4 to default to Low-High.
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Only values of 00 and 01 may be used |
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This change is hazardous, and should be made with care. The contents of location
958Bh are XOR'ed with the low memory address during all EPROM operations. Setting
any value other than 00 or 01 will scramble your data in complicated ways.
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